Monday, July 27, 2009

Symmetrical Investigation

Today (Monday), I wanted to investigate the skewness of the x-direction lateral scan that I have been discussing for the past few weeks. The easiest way for me to do this was to switch the angle of incidence of the excitation beam, because, if the beam was skewed as a result of the angle of the incident light in the x-direction, then we would expect the exact opposite skewness with the exact opposite angle of incidence. In quiet simple terms, I did exactly this... I switched the angle of incidence so we would in fact see a change in skewness if this was in fact the case.

Before switching the angle and changing the setup too much, I took a scan along the x-direction and a scan along the y-direction for some kind of a reference after switching the setup (though I already had such scans, I wanted to check that nothing had changed much in the design). Plots of this are shown below.

The first plot is that of the x-direction, while the second plot is of the y-direction. We see very much the exact same features that we have been seeing in such x- and y-direction scans -- the x-direction is rather skewed while the y-direction is rather symmetrical and Gaussian looking. This is not much different from what I expected (if they were different, then that would be a problem with the overall technique).

In order to better compare these scans with the scans done in the earlier weeks on the same setup, I simply plotted the x-direction scans all on one plot and all the y-direction scans on another plot. There were three different scans -- one with the first lens I used which did not focus the beam too well, and then two scans with the new lens in place which gave a tighter beam. These plots are shown below.

The first plot is that of the x-direction, while the second is that of the y-direction. The green data correspond to the scans taken with the first lens and at a larger step size (0.25 mm vs. 0.20 mm). This is most likely the reason why the data from this scan looks so much cleaner in a certain sense -- it was less precise. For the x-direction scans, it is clear that there is the same skewness in each scan (though it is a bit harder to see in the "scan 1"). It is especially clear in the difference between "2nd lens (1)" and "2nd lens (2)", as the only real difference is that these scans were taken on different days, which probably explains the difference in amplitude. All of the same sort of features show up in all of theses scans (and especially those of the 2nd lens). Therefore, it might be safe to assume that this method is in fact accurate to a certain extent.

The y-direction scans need much less explaining -- we see a nice, Gaussian shape in almost all of the scans and the main difference becomes the relative amplitude of the scans, though, again, this can (probably) be attributed to the change in laser power over the course of the day.

In fact, I did not take any scans with the new incidence today, but I did get the stage and lens moved to the proper position. I hope to take these scans tomorrow.

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